Apparatus for detecting and treating ventricular arrhythmia

ABSTRACT

A system and method for long-term monitoring of cardiac conditions such as arrhythmias is disclosed. The invention includes a pulse generator including means for sensing an arrhythmia. The pulse generator is coupled to at least one subcutaneous electrode or electrode array for providing electrical stimulation such as cardioversion/defibrillation shocks and/or pacing pulses. The electrical stimulation may be provided between multiple subcutaneous electrodes, or between one or more such electrodes and the housing of the pulse generator. In one embodiment, the pulse generator includes one or more electrodes that are isolated from the can. These electrodes may be used to sense cardiac signals.

FIELD OF THE INVENTION

[0001] This invention relates to a method and apparatus for treatingventricular arrhythmias; and more particularly, relates to a method andapparatus for long-term monitoring of arrhythmias, and for the deliveryof acute tachyarrhythmia and bradyarrhythmia therapy using asubcutaneous stimulation device.

DESCRIPTION OF THE PRIOR ART

[0002] It has long been known to use implantable systems to protectpatients that are at risk for life-threatening arrhythmias. For example,rapid heart rhythms commonly referred to as tachyarrhythmias aregenerally treated using implantable devices such as the Medtronic Model7273 GEM II DR or the 7229 GEM II SR, both commercially available fromthe Medtronic Corporation. These systems detect the presence oftachyarrhythmia conditions by monitoring the electrical and mechanicalheart activity (such as intra-myocardial pressure, blood pressure,impedance, stroke volume or heart movement) and/or the rate of theelectrocardiogram. These devices require that one or more defibrillationelectrodes be positioned within the atrium and/or ventricle of apatient's heart using current endocardial lead placement techniques. Theuse of such systems provides consistent long-term monitoringcapabilities, and relatively good protection against life-threateningtachyarrhythmias.

[0003] Similarly, bradyarrhythmias, which are heart rhythms that are tooslow, are generally treated using implantable pulse generators. Suchdevices are described in U.S. Pat. Nos. 5,158,078, 4,958,632, and5,318,593, for example. As with devices to treat tachyarrhythmias, mostimplantable pulse generators that treat these types of conditionsgenerally require leads that are implanted within one or more cardiacchambers.

[0004] Although the use of endocardial leads placed within the cardiacchambers of a patient's heart provides the capability to deliver arelatively reliable, long-term arrhythmia therapy, there aredisadvantages associated with such treatments. The placement of theseleads requires a relatively time-consuming, costly procedure that is notwithout risks to the patient including infection, the possibility ofvascular perforation, and tamponade.

[0005] Moreover, some people are not candidates for endocardial leads.For example, patients with artificial mechanical tricuspid valves aregenerally not candidates for leads that extend from the night atrium,through this valve, to the right ventricle, as is the case with mostright ventricular endocardial leads. This is because the use of suchleads interfere with the proper mechanical functioning of the valves.Other patients that are not candidates for endocardial lead placementinclude those with occluded venous access, or patients with congenitalheart defects.

[0006] Patients that are contraindicated for endocardial lead placementmust often undergo a procedure to attach the lead to the externalsurface of the heart. This type of epicardial lead placement involves amore invasive procedure that requires a longer recovery time, makesfollow-up procedures very difficult, and is also associated withincreased patient risk, including an increased chance of contracting aninfection.

[0007] Another problem associated with both endocardial and epicardialleads involves patient growth. More specifically, a lead placed within achild's cardiac vasculature will likely need to be re-positioned orreplaced as the child matures. Such lead replacement procedures can bedangerous, especially when previously-placed leads are extracted ratherthan left in position within the body.

[0008] One alternative to endocardial and epicardial leads involvessubcutaneously-placed electrode systems. For example, in U.S. Pat. No.RE30,372 by Mirowski, et al., a defibrillation system employs aventricular endocardial electrode and a plate electrode mounted to theheart directly, subcutaneously, or to the skin to deliver high voltagetherapy to the patient. A similar lead system disclosed in U.S. Pat. No.5,314,430 to Bardy includes a coronary sinus/great vein electrode and asubcutaneous plate electrode located in the left pectoral region whichmay optionally take the form of a surface of the defibrillator housing.

[0009] What is needed, therefore, is a system and method that canprovide long-term monitoring for various types of arrhythmias, providepatient therapy when needed, and also overcome the problems associatedwith both endocardial and epicardial lead placement.

SUMMARY OF THE INVENTION

[0010] The current invention provides a system and method for long-termmonitoring for arrhythmias. The invention includes a pulse generatorincluding means for sensing an arrhythmia. The pulse generator iscoupled to at least one electrode or electrode array for providingelectrical stimulation to a patient. The stimulation may includecardioversion/defibrillation shocks and/or pacing pulses. The electricalstimulation may be provided between multiple electrodes, or between oneor more electrodes and the housing of the pulse generator. In oneembodiment, the pulse generator includes one or more electrodes that areisolated from the can. These electrodes may be used to sense cardiacsignals.

[0011] According to one embodiment of the invention, an apparatus isprovided for monitoring cardiac signals of a patient. The apparatusincludes a hermetically-sealed housing, sensing means included withinthe housing, and first and second electrode sets coupled to the sensingmeans. The first electrode set includes at least one electrode adjacentto a surface of the housing positionable proximate subcutaneous tissueat a first location in the patient's body. The second electrode set iscoupled to a connector on the housing and forms an electrode arraysubcutaneously-positionable in the patient's body at a locationdifferent from the first location.

[0012] According to another embodiment of the invention, a method oftherapy is provided. This method includes monitoring the patient'scardiac signals for a condition such as an arrhythmia, and hereafterdelivering a electrical therapy to a patient via a subcutaneouselectrode array is the condition is detected. Other aspects of theinvention will become apparent from the drawings and the accompanyingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 illustrates an exemplary subcutaneous electrode and pulsegenerator as may be used in accordance with the current invention.

[0014]FIG. 2 is a block functional diagram of an illustrative embodimentof a pulse generator that may be employed according to the presentinvention.

[0015]FIG. 3 is a top view of an electrode array 300 as may be used withthe current invention.

[0016]FIG. 4A is a side view of a pulse generator illustrating theorientation of electrodes A, B and C disposed on the device housing.

[0017]FIG. 4B is a side view of a pulse generator wherein at least oneof the electrodes extends away from the pulse generator via a leadextension.

[0018]FIG. 4C is a side view of a pulse generator wherein at least oneof the electrodes is located at a proximal end of a lead.

[0019]FIG. 4D is a side view of a pulse generator wherein multipleelectrodes are located on an edge of a device housing.

[0020]FIG. 4E is a side view of yet another embodiment of a devicehousing including an array of electrodes.

[0021]FIG. 4F is a side view of a device having a first alternativeshape.

[0022]FIG. 4G is a side view of a device having a second alternativeshape.

[0023]FIG. 5 is a timing diagram illustrating one embodiment of adetection method used during bradyarrhythmia monitoring.

[0024]FIG. 6 is a block diagram illustrating an electrode arraypositioned around a patient's side, with electrode coils extending tothe patient's back.

[0025]FIG. 7 is a block diagram illustrating an electrode arraypositioned on patient's back in a more superior position.

[0026]FIG. 8 is a block diagram illustrating an electrode arraypositioned around a patient's side, with coil electrodes extending tothe patient's back in a more posterior position.

[0027]FIG. 9 is a block diagram illustrating an electrode arraypositioned on a patient's back, and a second subcutaneous disk electrodepositioned on a patient's chest.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The current invention provides a system and method for long-termmonitoring for arrhythmias. The invention also provides acute therapydelivery in the event an arrhythmia episode is detected. According toone embodiment of the invention, a subcutaneous pulse generator isprovided. This pulse generator may be a transthoracic ImplantableCardioversion/Defibrillator (ICD) such as the GemDR Model 7271 or theGEM II VR Model 7229, both commercially available from the MedtronicCorporation. The pulse generator is coupled to at least onesubcutaneously-placed electrode or electrode array.Cardioversion/defibrillation pulses and/or pacing pulses may bedelivered between the electrode and the can of the device, or betweentwo subcutaneously-placed electrodes.

[0029]FIG. 1 illustrates an implantable pulse generator 10 and anexemplary associated lead system according to the current invention.Pulse generator 10 includes a device housing 12, and is further coupledto a lead 14 which may be implanted subcutaneously in the left chest oron the back as discussed below. Lead 14 may include a subcutaneous plateelectrode 16, which may be any of the various known subcutaneous plateelectrodes. This type of subcutaneous electrode may be located proximalthe left ventricular-cavity on the patient's chest, on the patient'sside or back, or any other portion of the body appropriate for providingelectrical stimulation to the heart. Similar electrodes are disclosed inU.S. Pat. Nos. 4,392,407, 5,261,400, and 5,292,338, all incorporatedherein by reference. During use, electrical stimulation may be deliveredto heart 18 between electrode 16 and device housing 12.

[0030]FIG. 2 is a block functional diagram of an illustrative embodimentof a pulse generator that may be employed according to the presentinvention. As illustrated, the device is embodied as amicroprocessor-based stimulator. However, other digital circuitryembodiments and analog circuitry embodiments are also believed to bewithin the scope of the invention. For example, devices having generalstructures as illustrated in U.S. Pat. No. 5,21,624 issued to Bocek etal., U.S. Pat. No. 5,209,229 issued to Gilli, U.S. Pat. No. 4,407,288,issued to Langer et al, U.S. Pat. No. 5,662,688, issued to Haeffier etal., U.S. Pat. No. 5,855,893, issued to Olson et al., U.S. Pat. No.4,821,723, issued to Baker et al. or U.S. Pat. No. 4,967,747, issued toCarroll et al., all incorporated herein by reference in theirentireties, may also be usefully employed in conjunction with thepresent invention. FIG. 1 should thus be considered illustrative, ratherthan limiting with regard to the scope of the invention.

[0031] The primary elements of the apparatus illustrated in FIG. 2 are amicroprocessor 100, read-only memory (ROM) 102, random-access memory(RAM) 104, a digital controller 106, an input amplifier circuit 110, twooutput circuits 108 and 109, and a telemetry/programming unit 120.Read-only memory stores software and/or firmware for the device,including the primary instruction set defining the computationsperformed to derive the various timing intervals employed by the device.RAM 104 generally serves to store variable control parameters, such asprogrammed pacing rate, programmed cardioversion/defibrillationintervals, pulse widths, pulse amplitudes, and so forth which areprogrammed into the device by the physician. Random-access memory 104also stores derived values, such as the stored time intervals separatingtachyarrhythmia pulses and the corresponding high-rate pacing interval.

[0032] Controller 106 performs all of the basic control and timingfunctions of the device. Controller 106 includes at least oneprogrammable timing counter, which is used to measure timing intervalswithin the context of the current invention. On time out of the pacingescape interval or in response to a determination that a cardioversion,defibrillation, or pacing pulse is to be delivered, controller 106triggers the appropriate output pulse from high-voltage output stage108, as discussed below. In one embodiment, controller may also controlthe amplitude of pacing pulses, as well as the energy associated withdefibrillation and cardioversion shocks.

[0033] Following generation of stimulus pulses, controller 106 may beutilized to generate corresponding interrupts on control lines 132 tomicroprocessor 100, allowing it to perform any required mathematicalcalculations, including all operations associated with evaluation ofreturn cycle times and selection of anti-tachyarrhythmia therapiesaccording to the present invention. The timing/counter circuit incontroller 106 also may control timing intervals such as ventricularrefractory periods, as is known in the art. The time intervals may bedetermined by programmable values stored in RAM 104, or values stored inROM.

[0034] Controller 106 may also generate interrupts for microprocessor100 on the occurrence of sensed ventricular depolarizations or beats.The timing and morphology of sensed cardiac waveforms may also be usedby microprocessor 100 to determine whether an arrhythmia is occurring sothat therapy may be delivered as discussed further below.

[0035] Output stage 108 contains a high-output pulse generator capableof generating cardioversion/defibrillation pulses. According to thecurrent invention, these pulses may be applied between a subcutaneouselectrode or electrode array coupled to terminal 134 and the can of thepulse generator. Alternatively, the pulses may be provided between anelectrode coupled to terminal 134 and a second subcutaneous electrode orelectrode array coupled to terminal 136. Typically the high-output pulsegenerator includes one or more high-voltage capacitors, a chargingcircuit, and a set of switches to allow delivery of monophasic orbiphasic cardioversion or defibrillation pulses to the electrodesemployed. Output circuit 108 may further provide pacing pulses to theheart under the control of controller 106. These pacing pulses, whichmay be between 0 and 10 volts in amplitude, are provided via one or moreof the subcutaneously-located electrodes.

[0036] Sensing of ventricular depolarizations (beats) is accomplished byinput circuit 110, which is coupled to electrode 138 and one ofelectrodes 140 and 142. This circuitry may include amplification, andnoise detection and protection circuitry. In one embodiment, signalsensing is disabled during periods of excessive noise. Noise rejectionfilters and similar circuitry may also be included, as is known in theart. Input circuit 110 provides signals indicating both the occurrenceof natural ventricular beats and paced ventricular beats to thecontroller 106 via signal lines 128. Controller 106 provides signalsindicative of the occurrence of such ventricular beats to microprocessor100 via signal lines 132, which may be in the form of interrupts. Thisallows the microprocessor to perform any necessary calculations or toupdate values stored in RAM 104.

[0037] Optionally included in the device may be one or moresubcutaneously or cutaneously-positioned physiologic sensors 148, whichmay be any of the various known sensors for use in conjunction withimplantable stimulators. Any sensor of this type known in the art may beemployed within the context of the current invention. Additionally, ifdesired, sensors positioned within the cardiovascular system may beutilized. For example, sensor 148 may be a hemodynamic sensor such as animpedance sensor as disclosed in U.S. Pat. No. 4,86,036, issued toChirife or a pressure sensor as disclosed in U.S. Pat. No. 5,330,505,issued to Cohen, both of which are incorporated herein by reference intheir entireties. Alternatively, sensor 148 may be a demand sensor formeasuring cardiac output parameters, such as an oxygen saturation sensordisclosed in U.S. Pat. No. 5,176,137, issued to Erickson et al. or aphysical activity sensor as disclosed in U.S. Pat. No. 4,428,378, issuedto Anderson et al., both of which are incorporated herein by referencein their entireties.

[0038] Sensor processing circuitry 146 transforms the sensor output intodigitized values for use in conjunction with detection and treatment ofarrhythmias. These digitized signals may be monitored by controller 106and microprocessor 100 and used alone or in combination with sensedelectrical cardiac signals to provide diagnostic information used todetermine the onset of an arrhythmia or other cardiac conditions. Thesesignals may also be used to determine an optimal time for shockdelivery. For example, an impedance sensor may be used to determine whena patient has exhaled so that shock delivery may occur when the lungsare relatively deflated, since this may result in lower defibrillationthresholds (DFTs). Sensor signals may also be stored in RAM 104 forlater diagnostic use.

[0039] External control of the implanted cardioverter/defibrillator isaccomplished via telemetry/control block 120 that controls communicationbetween the implanted cardioverter/pacemaker and an external device 121.Any conventional programming/telemetry circuitry is believed workable inthe context of the present invention. Information may be provided to thecardioverter/pacemaker from the external device and passed to controller106 via control lines 130. Similarly, information from the cardcardioverter/pacemaker may be provided to the telemetry block 120 viacontrol lines 130 and thereafter transferred to the external device.

[0040] In one embodiment, the external device 121 is a programmer thatmay be utilized to diagnose patient conditions and to provide anynecessary re-programming functions. In another embodiment, the externaldevice may be a patient interface used to provide information to, and/orreceive commands from, the patient. For example, the patient interfacemay be an externally-worn device such as a wrist band that provides awarning to a patient concerning an impending shock. The patient may beallowed to cancel the shock if the patient believes the shock wasprescribed erroneously. This may be accomplished, for example, bypushing a button, or issuing a voice command. The patient interface mayprovide additional information, including a warning that medicalattention is required, and/or an indication concerning a low powersource. If desired, the patient interface could automatically place anemergency telephone call via a wireless link, and/or could issue patientpositional information via a global positioning system (GPS).

[0041] Any other system and method used for the detection and treatmentof tachyarrhythmias may be incorporated within the current invention.Such systems and methods are described in U.S. Pat. Nos. 5,849,031,5,193,535, and 5,224,475. In one embodiment the system may include“tiered therapies” for delivering treatment based on the type ofarrhythmia detected by the device. According to this approach,arrhythmias are differentiated by analyzing the rate and morphology of asensed cardiac signal. Those arrhythmias considered less dangerous suchas ventricular tachycardias (VTs) may be treated by delivering a seriesof low-power, relatively high-rate, pacing pulses to the heart. Thistherapy is often referred to as anti-tachyarrhythmia pacing therapy(ATP). In contrast, more perilous arrhythmias such as ventricularfibrillations (VFs) may be treated by immediately delivering moreaggressive shock therapy. This type of system is described in U.S. Pat.No. 5,193,536, issued to Mehra, U.S. Pat. No. 5,458,619 to Olson, U.S.Pat. No. 6,167,308 to DeGroot, and U.S. Pat. No. 6,178,350 to Olson, etal., all incorporated herein by reference. Within the context of thecurrent invention, ATP therapy is delivered using one or moresubcutaneous electrodes in the manner discussed below. In one embodimentof the invention, a separate electrode may be provided within asubcutaneous electrode array for delivering the ATP therapy.

[0042] According to another aspect of the inventive system, the devicemay include means for decreasing discomfort associated with high-voltageshocks. It is well known that high-voltage shocks are painful for thepatient. This discomfort can be minimized by decreasing the amount ofenergy associated with the shock. One mechanism for accomplishing thisinvolves delivering a pre-shock pulse waveform, as described in U.S.Pat. No. 5,366,485 issued to Kroll. In one embodiment, this type ofwaveform could be a programmable feature that is controlled bycontroller 106 via parameters stored in RAM 104.

[0043] In yet another embodiment of the invention, the implantabledevice includes a drug pump 10 as shown in FIG. 2. This pump may be usedto deliver a biologically-active agent such as an analgesic drug to thepatient prior to shock delivery to reduce discomfort. The drug deliverymay be accomplished via a catheter 12 that is implanted subcutaneouslyor within the patient's vascular system. A similar system is describedin U.S. Pat. No. 5,893,881 to Elsberry, incorporated herein byreference. Alternatively, or in addition, this pump may deliver an agentsuch as D-sotalol, Procainamide or Quinidine to reduce thedefibrillation threshold of the required shock, thereby serving toreduce pain. In a more complex embodiment, two separate drug pumps mightbe employed to allow delivery of the threshold reducing agent alone orin conjunction with an analgesic.

[0044] Pain control may also be accomplished by providing spinal cordstimulation (SCS). For example, the Medtronic Itrel II implantableneurostimulation system is widely implanted for treatment andalleviation of intractable pain. Clinical reports and studies have shownthat SCS can reduce the discomfort associated with high-voltage shocks.This type of system may utilize a lead system of the type described inU.S. Pat. No. 5,119,832, 5,255,691 or 5,360,441. These leads, as well asthe Medtronic Model 3487A or 3888 leads, include a plurality of spacedapart distal electrodes that are adapted to be placed in the epiduralspace adjacent to spinal segments T1-T6 to provide SCS stimulation forpain reduction. In this embodiment, initial detection and verificationof fibrillation is followed by epidural neural stimulation to produceparaesthesia. Thereafter, a shock may be delivered. Should thecardioversion shock prove unsuccessful, the process is repeated untilthe cardioversion therapies prove successful or are exhausted. Whensuccessful defibrillation is confirmed, the epidural SCS stimulation ishalted.

[0045] In addition to SCS therapy, other types of stimulation such asTranscutaneous Neurological Stimulators (TENs) may be provided viaelectrode patches placed on the surface of a patient's body.Subcutaneously-placed electrodes may also be positioned in the T1-T6area or in other areas of the body to deliver subcutaneous electricalstimulation to reduce pain. In the context of the current invention, thesubcutaneously-placed electrode arrays may include specializedelectrodes to deliver the subcutaneous stimulation prior to shockdelivery to reduce patient discomfort.

[0046] Turning now to a more detailed discussion of the electrodesystems used with the current invention, the electrode may be of a typeshown in FIG. 1. Alternatively, this electrode array may be similar tothe Model 6996 SQ commercially-available from the Medtronic Corporation.

[0047]FIG. 3A is a top view of an electrode array 300 as may be usedwith the current invention. Electrode array 300 is coupled to distal endof lead 302. The array includes multiple finger-like structures 304Athrough 304E. More or fewer of these finger-like structures may beprovided. Each finger includes a defibrillation coil electrode shown as306A through 306E. When connector 308 is coupled to a pulse generator, acardioversion/defibrillation pulse may be provided via one or more ofthe electrodes 306A through 306E. In one embodiment, the electrodes thatare activated may be selected via a switch provided by the lead.

[0048] Electrode array 300 may include one or more sensing electrodessuch as electrode 310 provided for sensing cardiac signals. Thiselectrode may be used in a unipolar mode wherein signals are sensedbetween an electrode and the device housing. Alternatively, sensing maybe performed between electrode 310 and one of the coil electrodes 306 oranother sensing electrode.

[0049] In use, the fingers 304 of electrode array are positioned underthe skin on a patient's chest, side, back, or any other point of thebody as required. Insulative spacers may be located between the fingers,if desired, to prevent the coil electrodes 306A-E from shortingtogether. If desired, multiple such electrode arrays may be used inconjunction with the current invention. For example, one electrode arraymay be positioned on the chest over the left ventricle, while anotherelectrode array is positioned behind the left ventricle on the back.Cardioversion/defibrillation shocks or pacing pulses may be deliveredbetween the two electrode arrays. Alternatively, electrical stimulationmay be provided between one or more electrode arrays and the devicehousing. As noted above, sensing of the patient's cardiac signals may beperformed between a subcutaneous electrode array and the device can.

[0050]FIG. 3B is a top view of an alternative embodiment of electrodearray, shown as array 300A. In this embodiment, fingers 320A through320C have a serpentine shape. More or fewer such fingers may beprovided. This shaped array directs current provided by coiledelectrodes 322A through 322C through a larger tissue area, therebydecreasing defibrillation thresholds in some instances. This embodimentmay also include one or more sensing electrodes 322. Any other shape maybe utilized for the electrode array.

[0051] The electrodes used with the current invention may be any of theelectrode types now known or known in the future for subcutaneousdelivery of electrical stimulation. Such electrodes may be coated with abiologically-active agent such as glucocorticolds (e.g. dexamethasone,beclamethasone), heparin, hirudin, tocopherol, angiopeptin, aspirin, ACEinhibitors, growth factors, oligonucleotides, and, more generally,antiplatelet agents, anticoagulant agents, antimitotic agents,antioxidants, antimetabolite agents, and anti-inflammatory. Such coatingmay be useful to prevent excessive tissue in-growth. Such electrodes mayfurther include a low-polarization coating such as TiN. Alternatively,the electrodes may be coated with an antibiotic or otherbiologically-active agent used to prevent infections and inflammation.

[0052] In another embodiment, the can itself may include a subcutaneouselectrode array of the type described in U.S. Pat. No. 5,331,966, whichis incorporated herein by reference in its entirety. This type of array,which is provided by the Medtronic Model 926 Reveal Plus ImplantableLoop Recorder, includes at least two sensing electrodes on the can forsensing of cardiac signals. In all such systems, it will be understoodthat the electrodes A, B, C on the surface of the housing areelectrically isolated from one another and the conductive surface of thepulse generator housing 10 through suitable insulating bands andelectrical feed throughs as described in U.S. Pat. No. 4,310,000,incorporated herein by reference. Examples of possible electrodeorientations and configurations of a three electrode system comprisingthe electrodes are set forth in FIGS. 4A through 4G.

[0053]FIG. 4A is a side view of a pulse generator illustrating theorientation of orthogonally-disposed electrodes A, B and C with twoelectrodes on the connector block 418 and one electrode on the pulsegenerator case 410. The spacing of the electrodes A, B and C on each ofthe illustrated orientations of FIGS. 4A through 4G may be on the orderof about one inch but can be larger or smaller depending on the exactsize of the device. Smaller devices and closer spacing will requiregreater amplification.

[0054]FIG. 4B is a side view of a pulse generator wherein at least oneof the electrodes extends away from the pulse generator by a leadextension member 420 to achieve a greater inter-electrode spacing, ifdesirable.

[0055]FIG. 4C is a side view of a pulse generator wherein at least oneof the electrodes 230 is located at a proximal end of a lead 432, whichmay be a lead coupled at a distal end to a subcutaneous electrode orelectrode array.

[0056]FIG. 4D is a side view of a pulse generator wherein multipleelectrodes are located of an edge of a device housing. It will beunderstood that the electrodes placed on the edge of the pulse generatorcase could constitute insulated pins of feedthroughs extending throughthe wall of the case. As illustrated in FIGS. 4C and 4D, the relativeorientation of the electrodes may vary somewhat from the orthogonalorientation depicted in FIGS. 4A and 4B.

[0057]FIG. 4E is a side view of yet another embodiment of a devicehousing including an array of electrodes.

[0058]FIG. 4F is a side view of a device having a first alternative “T”shape. This shape allows at least two of the electrodes A and C to bepositioned at a maximum distance from one another, optimizing signalreception between the two electrodes.

[0059]FIG. 4G is a side view of a device having a second alternative“boomerang” shape which may be used to optimize electrode positioning sothat better signal reception is achieved.

[0060] It will be appreciated that the shapes, sizes, and electrodeconfigurations of the devices shown in FIGS. 4A through 4G are exemplaryonly, and any other shape, size or electrode configuration imaginable iswithin the scope of the current invention. As will be appreciated bythose skilled in the art, those configurations allowing for greaterinter-electrode distances will generally provide better signalreception. As such, it is usually desirable to provide electrodes on atleast two quadrants of the device.

[0061] As described above, in one embodiment, the current inventionprovides a pulse generator coupled to one or more subcutaneouselectrodes or electrode arrays. The electrodes provide electricalstimulation to a patient based on sensed cardiac signals. The sensedsignals may be obtained using a selected pair of sensing electrodes,which may reside on one or more of the leads coupled to pulse generator10, or on the device housing itself, as indicated by FIGS. 4A through4G.

[0062] Although all of the foregoing examples illustrate a housingincluding three electrodes, more than three electrodes may be provided.In one embodiment, four or more electrodes may be coupled or adjacent tothe device, and the physician may select which of the electrodes will beactivated for a given patient. In one embodiment, cardiac signals aresensed between a selected pair of the electrodes based on a signaloptimization method. One embodiment of this type of method is disclosedin U.S. patent application Ser. No. 09/721,275 filed Nov. 22, 2000 andincorporated herein by reference in its entirety.

[0063] Regardless of which one or more electrodes or electrode pairs areselected for monitoring purposes, the sensed cardiac signals may beanalyzed to detect the presence of an arrhythmia. The arrhythmiadetection system and method could be, for example, that employed by theMedtronic Model 926 Reveal Plus device commercially available fromMedtronic Corporation. Alternatively, a detection method such asdescribed in U.S. Pat. No. 5,354,316 or 5,730,142 could be employed. Ifan arrhythmia is detected, appropriate therapy may be administered. Asdescribed above, one embodiment of the invention includes at least onesubcutaneous defibrillation electrode array. If monitoring indicates thepresence of a tachyarrhythmia or ventricular fibrillation, ahigh-voltage shock may be delivered between one or more subcutaneousdefibrillation electrode(s) and a shocking surface of the can, or one ormore electrodes on the can. The shock may alternatively be deliveredbetween multiple defibrillation electrodes. The monitoring system wouldthen determine whether the arrhythmia or fibrillation has terminated. Ifnot, another shock will be administered. This therapy will continueuntil normal rhythm has been restored. In one embodiment, signalsindicative of sensed cardiac waveforms may be stored in RAM 104 andlater transferred to an external device via a communication system suchas telemetry circuitry 120.

[0064] According to another aspect of the invention, the sensingelectrodes may be placed on a surface of the can that is different fromthe shocking surface of the can. Preferably, the shocking surface isadjacent to muscle tissue, whereas the sensing electrodes are placedadjacent to subcutaneous tissue.

[0065] As described above, therapy for bradyarrhythmia may be providedin addition to, or instead of, the tachyarrhythmia therapy. In thisembodiment, output circuit 108 includes the capability to deliverlower-voltage pulses for transthoracic pacing therapy forbradyarrhythmias, as described above in reference to FIG. 1. Theselower-voltage pulses could be on the order of between 0 and 10 volts,for example. In one embodiment, these pulses have an amplitude of around100 volts. Monitoring for a bradyarrhythmia could be accomplished usingthe sensing electrodes discussed above. For example, the device may beprogrammed to detect a period of asystole that is greater than apredetermined period, such as three seconds. When a period greater thanthis length is detected, the output circuit of the device is charged tothe pacing voltage. A transthoracic, monophasic pacing pulse may then bedelivered between the shocking surface of the can and a subcutaneouselectrode or electrode array, or between two such electrode or electrodearrays. The sensing electrodes monitor the cardiac waveform to ensurethat the pacing pulse is only delivered during predetermined periods ofthe cardiac cycle. For example, delivery of the pulse should not occurduring the occurrence of a T-wave.

[0066] Following delivery of a pacing pulse, the output circuit beginscharging in preparation for delivery of another pulse while monitoringof the cardiac signals continues. For example, monitoring of thepatient's heart rate may be performed to determine whether it is lessthan some predetermined rate such as forty beats per minute. If so,another transthoracic, monophasic pacing pulse is delivered. Thisprocess of pulse delivery followed by charging of the output circuit isrepeated until an intrinsic heart rate of greater than the predeterminedminimum rate is detected.

[0067] The transthoracic pacing provided by the current invention willlikely be uncomfortable for the patient. Thus, this function is notintended to provide chronic therapy. Once therapy delivery has occurredfor a bradyarrhythmic episode, a more traditional device should beimplanted to provide long-term therapy. In one embodiment, the devicemay record whether any ACC/AHA class I pacing indications has been metby the detected bradyarrhythmic event. For example, if asystole greaterthan three seconds and/or an escape rate less than forty beats perminute has been detected, these indications are recorded. This data maythen be transferred to an external device to generate a physiciannotification. Other actions may be taken, such as sounding an alarm, forexample.

[0068]FIG. 5 is a timing diagram illustrating one embodiment of adetection method used during bradyarrhythmia monitoring. If asystole isdetected for greater than, or equal to, a first predetermined, timeperiod 500 such as three seconds, charging of output capacitors occursto a predetermined voltage such as 100 volts. This charging occursduring time period 502. At time 504, a first pacing pulse is delivered,and recharging of the capacitors begins at time 506. Monitoring for anescape rate longer than a predetermined rate occurs during time period508, which in one embodiment is 100 milliseconds. Thereafter, a secondpacing pulse is delivered at time 510 if an intrinsic beat does notoccur. At time 512, recharging occurs, and monitoring for the escaperate again proceeds. If such therapy is not discontinued because of there-occurrence of the patient's intrinsic normal heart beat, the patientwill be required to seek immediate emergency attention, since suchtherapy will be uncomfortable for the patient. The times utilized toprovide therapy as shown in FIG. 5 may be programmable.

[0069] It may be appreciated from the foregoing discussion thatproviding repeated therapy, and in particular, repeated high-voltagepacing stimulation, will deplete a system power source, such as abattery, relatively quickly. Therefore, in one embodiment, the powersource is rechargeable. For example, the pulse generator may includerechargeable nickel cadmium batteries. Such batteries may be rechargedover a period of several hours using a radio frequency link.Alternatively, a rechargeable capacitive energy source such as disclosedin U.S. Pat. No. 4,408,607 to Maurer may be utilized. In yet anotherembodiment, the pulse generator may include both an implanted radiofrequency (RF) receiving unit (receiver) incorporating a back-uprechargeable power supply and a non-rechargeable battery, as describedin U.S. Pat. No. 5,733,313 incorporated herein by reference. Therechargeable power supply is charged by an external RF transmitting unitworn by the patient. Any other type of rechargeable power supply knownin the art for use with implantable medical devices may be used in thealternative.

[0070] In one embodiment, the power source selected for use in thecurrent invention is capable of delivering up to ten therapy shocks,with additional power being available for threshold testing. However,compromises will exist since the power source capacity will determinedevice size. In yet another embodiment the device is a 75-joule devicehaving a volume of no more than 75 cubic centimeters. Preferably, thedevice includes a power source and associated charge circuitry thatprovides a charge time of no more than three minutes during the usefullife of the device. In another embodiment, the device should be capableof delivering a 35-joule shock after a one-minute charge time over theuseful life of the device.

[0071]FIGS. 6 through 9 illustrate various exemplary electrodeconfigurations as may be used with the current invention.

[0072]FIG. 6 is a block diagram illustrating an electrode array 300positioned around a patient's side, with fingers 304 extending to thepatient's back. Electrical stimulation is delivered between theelectrode array and the device can 10, which is positioned over the leftventricle. In one embodiment, sensing electrodes 600 are positionedsubstantially facing toward subcutaneous tissue.

[0073]FIG. 7 is a block diagram illustrating an electrode arraypositioned on a patient's back in a more superior position than is shownin FIG. 6. Electrical stimulation is delivered between the electrodearmy and the device can 10, which is positioned in the abdominal cavity.

[0074]FIG. 8 is a block diagram illustrating an electrode arraypositioned around a patient's side, with fingers 304 extending to thepatient's back in a more posterior position than is shown in FIG. 6 or7. Electrical stimulation is delivered between the electrode array andthe device can, which is positioned proximal the right-side of theheart.

[0075]FIG. 9 is a block diagram illustrating an electrode array withfingers 304 positioned on a patient's back, and a second subcutaneousdisk electrode 306 such as electrode 16 (FIG. 1) positioned in apatient's chest. Electrical stimulation may be delivered from one ofelectrodes 304 or 306 to another electrode and/or the device housing 10.Alternatively, stimulation may be provided from both electrodeassemblies to the device housing. In yet another embodiment, one or moreadditional subcutaneous electrode or electrode arrays may be coupled tothe device for providing high-voltage shocks, for sensing cardiacsignals, and/or for delivering SCS, TENs, or subcutaneous low-voltagestimulation as discussed above. If desired, the device may includeprogrammable logic to selectably enable those electrode and/or electrodearrays to be activated during a given therapy delivery session. Forexample, switching networks may be incorporated into output circuitry108 and/or input circuitry 110 (FIG. 2) such that this type ofprogrammably selected therapy may be provided. In one instance, it maybe desirable to activate one electrode configuration to optimize sensingof cardiac signals, while utilizing another configuration to provideoptimal therapy delivery.

[0076] The above-described inventive system and method provides atherapy that avoids the risks of transvenous lead delivery. Such asystem may be used for patients that are at-risk for arrhythmias, buthave not yet experienced a confirmed arrhythmic episode. The device maytherefore provide a needed long-term monitoring function, as well as anyinterventional therapy that is required. Preferably, after an episode isdetected and therapy is delivered for a first time, the current systemwould be replaced with a more conventional implantable defibrillator.

[0077] As discussed above, the inventive system provides many importantbenefits over other conventional systems for some patients. Theprocedure is faster because there is no need for venous or epicardialaccess, and therefore the procedure is less invasive, and would notrequire procedures needing sophisticated surgical facilities anddevices. Additionally, the implant procedure can be accomplished withoutexposing the patient to potentially-harmful radiation that accompaniesfluoroscopy. The risk of infection is reduced, and the procedure may beprovided to patients that are contraindicated for a more traditionaldevice. Additionally, one hundred percent patient compliance isachieved, and the system is more comfortable than externally-worndevices. The system is well suited for pediatric use, since theplacement of the electrodes allows lead length to be easily extended asa patient grows. The system may also be employed in parts of the worldwhere more long-term therapies and treatments are not available, andwhere sophisticated surgical skills and equipment cannot be readilyobtained.

What is claimed is:
 1. A system for providing arrhythmia therapy to apatient comprising: an implantable pulse generator; a sensing circuitcoupled to the implantable pulse generator; and a subcutaneous electrodearray coupled to the implantable pulse generator to deliver electricalstimulation to the patient upon detection by the sensing circuit of anarrhythmia.
 2. The system of claim 1, wherein the subcutaneous electrodearray is a defibrillation electrode array to deliver relativelyhigh-voltage electrical stimulation to the patient.
 3. The system ofclaim 1, wherein the system is housed within a can, and wherein thesensing circuit includes at least two sensing electrodes on at least afirst surface of the can to sense cardiac signals.
 4. The system ofclaim 2, wherein the system is housed within a can, and wherein thesensing circuit utilizes the defibrillation electrode array and the canto sense for an arrhythmia.
 5. The system of claim 1, wherein the canincludes at least one surface to deliver high-voltage shocks.
 6. Thesystem of claim 3, wherein the can includes at least one surface todeliver high-voltage shocks, and wherein the surface to deliverhigh-voltage shocks is different from the at least first surface of thecan.
 7. A method for treating patient arrhythmias, including the methodsof: a) providing a subcutaneous pulse generator; b) providing amonitoring circuit to monitor the patient's cardiac signals forarrhythmias; and c) providing a subcutaneous electrode array to deliverelectrical therapy to a patient.
 8. A method of using asubcutaneously-placed pulse generator to treat arrhythmias, includingthe methods of: a) detecting an arrhythmia; and b) employing at leastone subcutaneous electrode to deliver therapy based on the detectedarrhythmia.
 9. The method of claim 8, wherein the detected arrhythmia isa tachyarrythmia, wherein the at least one subcutaneous electrodeincludes a defibrillation electrode, and wherein the therapy that isdelivered is a relatively high-voltage shock.
 10. A system fordelivering electrical energy to the heart of a patient, the systemcomprising: a subcutaneous pulse generator; at least one sensingelectrode disposed on a surface of the pulse generator and positionedproximate to subcutaneous tissue; and at least one electrode arraycoupled to the pulse generator and positioned subcutaneously on thepatient.
 11. An apparatus for monitoring cardiac signals of a patient,comprising; a hermetically-sealed housing; sensing means included withinthe housing; and first and second electrode sets coupled to the sensingmeans, the first electrode set including at least one electrode adjacentto a surface of the housing positionable proximate subcutaneous tissueat a first location in the patient's body, and the second electrode setcoupled to a connector on the housing and forming an electrode arraysubcutaneously positionable in the patient's body at a locationdifferent from the first location.